Antennas with high directional gains are widely used to improve power efficiency in line of sight wireless transmission. As a wireless signal propagates, regardless of the pattern of the transmit antenna, the propagating wave will gradually tend to a planar wave. In line of sight transmissions using directional antennas, only two polarizations may be used to provide two spatial channels. This results in a 2×2 Multiple-Input Multiple-Output (MIMO) channel. Typically, the two spatial dimensions correspond to horizontal and vertical linear polarizations, or correspond to left-handed and right-handed circular polarizations. In such systems, dual-polarized antennas are widely used. Further, in such line-of-sight transmission setups, it is desirable to have a non-singular channel matrix supporting more than two dimensions.
The number of spatial channels can be increased beyond two, at the cost of using multiple (separate) transmit and multiple (separate) receive antennas, with restrictions on spacing among transmit and/or among receive antennas. In such setups, as the receive antennas detect a plane wave, the channel matrix in the underlying MIMO system may be close to singular. Alternatively, non-line-of-sight MIMO structures may realize additional spatial dimensions without such severe restrictions on their spacing due to the effect of multi-path fading. The reason is that, in conventional MIMO systems (non-line-of-sight), the channel matrix is composed of (independent) complex random numbers, and consequently, the channel matrix will be (with high probability) non-singular. In prior art, to provide a non-singular channel matrix supporting more than two spatial dimensions in line-of-sight transmission setups, it is required to have a large spacing among dual-polarized transmit antennas and a large spacing among dual-polarized receive antennas. Thus it may be desirable to increase the number of spatial dimensions in line-of-sight wireless links without such severe restrictions on antenna spacing.